Thin film solar panels are gaining ever more recognition in the field of solar panels because of their relative low cost compared to panels of traditional technology using silicon wafers. Solar panels based on a photovoltaic structure using a semiconductor layer of cadmium telluride (CdTe) deposited on a glass sheet attain conversion costs smaller than $2.00 per watt and are rapidly approaching about $1.00 per watt.
Other thin film technologies of ternary or quaternary chalcogens are the so-called photovoltaic structures CIS (Copper Indium di-Selenide) and CIGS (Copper Indium Gallium di-Selenide) that promise conversion yields above 10% at very competitive costs.
Similar efficiencies values have been obtained using a thin film technology based on hydrogenated amorphous silicon (a-Si:H). Besides the smaller cost of metallurgic silicon as compared to the cost of elements, such as cadmium, indium, tellurium, there is the further advantage of not including toxic materials, such as, for example, cadmium. Thus, they would be more safely disposed of in the environment without using costly processes for recovering substances that cannot be dispersed in the environment, at the end of the working life of panels.
Among the numerous factors that may influence the choice of a thin film technology among the available technologies, it should be considered that among these thin film fabrication technologies of solar panels, there are important differences in functioning at the various wavelengths of the solar spectrum.
A hydrogenated amorphous silicon photovoltaic structure (p-i-n or n-i-p diode) has a relatively large band gap (slightly less than 2 eV) that makes the semiconductor particularly efficient in converting the blue portion of the solar spectrum, but not so in the red portion of the solar spectrum. Moreover, hydrogenated amorphous silicon tends to significantly “trap” carriers (in particular holes), thus keeping the thickness of the intrinsic semiconductor layer (i) (i.e. the absorption-conversion region) of the p-i-n or n-i-p structures within a few hundred nanometers. In addition, hydrogenated amorphous silicon undergoes an “aging” process, generally attributed to hydrogen desorption by structural defects of the material (the D-centers). All these characteristics may lead to the use of hydrogenated amorphous silicon as part of a relatively thin p-i-n or n-i-p structure, which is adapted to absorb blue-green radiation of the solar spectrum.
As far as the absorption exploitation of the red radiation part of the solar spectrum is concerned, it is either neglected for privileging a reduced cost of panels or done by a second tandem photovoltaic structure of a different semiconductor or by realizing stacked multi-junction photovoltaic structures. Semiconductors generally used for a tandem or multi-junction structure associated to a hydrogenated amorphous silicon structure, are microcrystalline silicon, hydrogenated silicon-germanium alloys (a-SiGe:H), ternary chalcogens (for example, CIS) or quaternary chalcogens (for example CIGS). See U.S. Pat. No. 6,368,892. Tandem structures of hydrogenated amorphous silicon and of microcrystalline silicon are known in the industry as micromorphic cells. These as well as other developments based on tandem or multi-junction structures are efficient, but up to now, they have lead to power conversion yields that at the most attain, at module level, 13% (see Martin A. Green, Keith Emery, Yoshihiro Hishikawa and Wilhelm Warta, Prog. Photovolt: Res. Appl. 2008; 16:61-67), i.e. well below the theoretical limit of the single photovoltaic structures combined together.
As it is known to the skilled person, certain characteristics of the thin film technology, with respect to other technologies, may negatively counterbalance certain positive characteristics of otherwise superior thin film technologies.
Micromorphic cells, that are promising from the point of view of cost and reliability, may suffer the drawback of a short-circuit current yield far lower than the theoretical limit. This is due to poor absorption capability of radiation in the red region of the solar spectrum. An increase of thickness of the microcrystalline intrinsic semiconductor material alleviates this problem, but it may imply a significant increase of costs.
By contrast, CIS and CIGS semiconductor structures are efficient even if in thin film form, in the order of 1.0 μm thickness. This is due to the fact that, differently from amorphous and microcrystalline silicon, these materials are direct band gap semiconductors capable of efficiently capturing photons even in very thin films. Up to now, the best results have been obtained with semiconductors, i.e. CIS or CIGS, having a low band gap, even if practical limits due to recombination of carriers in the region of spatial charge of the photovoltaic conversion structure are present.
As for amorphous silicon, even with these chalcogen materials it is possible to adjust the band gap value, for example, by exchanging Ga and In contents in CIGS. However, when increasing the band-gap from 1.0 to about 1.5 eV, a reduction of the power conversion yield by a factor of about 2 is observed. This indicates an intrinsic inefficiency of chalcogen photovoltaic cells in converting the blue portion of the solar spectrum.
An ideal structure appears to be a combination of a CIS, CIGS or CdTe converting structure and an amorphous silicon converting structure. The resulting combined photovoltaic structure efficiently converts both the red portion as well as the blue portion of the solar spectrum. See U.S. Pat. No. 6,368,892. Solar light crosses first an amorphous silicon structure photovoltaic diode that absorbs and converts radiation of wavelength in the blue region of the spectrum while the remaining green-red portion of the spectrum crosses the silicon diode, being only marginally absorbed, and it is efficiently absorbed and converted by an underlying CIS, CIGS or CdTe converting structure made of direct gap semiconductor material of a thickness sufficient to absorb substantially all the radiation reaching it.
Typically, this type of prior art conversion “stack” is monolithic and has the drawback that cells of the two photovoltaic conversion structures of different technology are electrically in series and thus crossed by the same current. This may significantly limit the overall power conversion yield.